1. Technical Field
The disclosure relates to an oscillator which includes a crystal unit and an oscillator circuit for generating a clock.
2. Description of the Related Art
Some oscillators may be configured as an oven controlled crystal oscillator (OCXO) 100 illustrated in FIG. 9. In the OCXO 100, a temperature in the oven is calculated using a difference between frequencies respectively output from two crystal units. Then, based on the calculated temperature, an output of a heater 46 is the corrected, and a frequency compensation value is changed, the frequency compensation value being output to a PLL circuit unit 41 from a temperature compensation frequency calculating unit 32 so as to reduce a frequency variation outputs of a voltage controlled crystal oscillator (VCXO) 43. The section of embodiments will describe the details of respective components in the OCXO 100, and then the section of the related art only outlines the respective components. Note that Japanese Unexamined Patent Application Publication No. 2013-51677 also discloses an OCXO with a configuration approximately similar to the disclosure.
In the OCXO 100, first oscillator circuits (OSC) 11, a second oscillator circuit 21, and a digital control circuit 33 or similar circuits are included in a common integrated circuit (LSI), which is a digital signal processing unit 3. The LSI uses an oscillation frequency of the first oscillator circuit 11 as a system clock. Incidentally, the oscillation characteristics of the crystal unit vary for each crystal unit. Then, an oscillator circuit causing a crystal unit to oscillate is configured to change oscillation parameters such that the oscillator circuit can cause respective crystal units to oscillate at a frequency within the predetermined range even if respective crystal units have such a variation in oscillation. The oscillation parameter is a circuit constant of the oscillator circuit, and the setting of the oscillation parameter determines a current value which flows through the oscillator circuit and a capacitance value and an inductor value of a capacitor which constitutes the oscillator circuit or a similar value.
The LSI of the OCXO 100 stores initial parameters, which are set at each component in the LSI when the OCXO 100 is powered to activate the LSI, the initial parameters including the oscillation parameter. Namely, the OCXO 100 preliminary determines the oscillation parameters to be set for the first oscillator circuit 11 when the OCXO 100 is powered, and the initial parameters will be standard oscillation parameters.
The description will continue with reference to a conceptual diagram of FIG. 10. A reference numeral A in the drawing indicates a frequency range of a clock of the LSI. The LSI can be activated by supplying a clock, which is in the frequency range A, when the OCXO 100 is powered. The activated LSI appropriately sets the parameters of the circuits which constitutes the LSI, which allows the OCXO 100 to output a signal with desired frequency. A reference numeral B in the drawing indicates a frequency range (changeable range) within which the LSI can cause the crystal unit to oscillate. The crystal unit which oscillates at frequencies within the changeable range B can change the oscillation parameters of the first oscillator circuit 11 from the standard oscillation parameters to other oscillation parameters so as to change its oscillation frequency to an oscillation frequency within the clock frequency range A.
When the standard oscillation parameter is set, the LSI of a crystal unit C which is illustrated in FIG. 10 receives an appropriate clock (clock in frequency range A) to oscillate at a frequency within the changeable range B and the clock frequency range A. Accordingly, the crystal unit C can be used as the first crystal unit 10.
A crystal unit D illustrated in FIG. 10 oscillates at a frequency within the changeable range B but not within the clock frequency range A. Here, if the LSI operates with being supplied with an appropriate clock, the LSI can change the standard oscillation parameters of the first oscillator circuit 11 to, for example, the oscillation parameters read out from outside of the LSI, and can cause the crystal unit D to oscillate at a frequency within the clock frequency range A. On the contrary, if the LSI does not operate because no clock is supplied, the LSI cannot change the oscillation parameters, which makes the crystal unit D useless as the first crystal unit 10.
A description will be given with particular examples, in which the clock frequency range A for the LSI is from 50 MHz to 90 MHz. Then, the first crystal unit 10 oscillates, with the standard oscillation parameter, at an oscillation frequency of 20 MHz. If the first crystal unit 10 can oscillate at a frequency of 60 MHz using a triple wave, the first crystal unit 10 can supply the appropriate clock to the LSI to activate the LSI. The standard oscillation parameter, however, is determined in advance, which makes it impossible for the first crystal unit 10 to oscillate at such a frequency of 60 MHz.
In order to address the above-described situation, the OCXO 100 may be configured with the first crystal unit 10 which can oscillate at either frequency of 20 MHz and 60 MHz when the OCXO 100 is powered. In particular, for example, an external circuit is provided at the outside of the LSI via which the crystal unit is connected to the oscillator circuit, the external circuit adjusting the capacitance component and the inductor component between the crystal unit and the oscillator circuit. This, however, results in a large circuit size. One option is developing a new LSI which can oscillate at 20 MHz and 60 MHz. This, however, requires large cost.
The above described is a case in which an output of the first oscillator circuit 11 is used as a system clock of the LSI. The similar problem arises when an output of the second oscillator circuit 21 is used as a system clock. In other word, the crystal units are required to be selected as the first crystal unit 10 or as the second crystal unit 20 such that the crystal units can oscillate at a frequency within frequency range of the system clock of the LSI, therefore the available types and combinations of the crystal units are unfortunately limited. Japanese Unexamined Patent Application Publication No. 2001-344039 and Japanese Unexamined Patent Application Publication No. 2012-208804 disclose techniques that switch clocks of the systems, but they do not disclose the above-described problems.
The disclosure has been made in view of such circumstances, and an aim thereof is to provide a technique which increases selectable types and combinations of crystal units used in an oscillator which includes a crystal unit and an oscillator circuit for generating a clock.